1. Field of the Invention
The present invention relates to a radio communication system including a radio base station and a radio network controller, a radio base station, a radio network controller and a radio communication method used in a radio communication system.
2. Description of the Related Art
Conventionally, a radio communication system including a radio base station and a radio network controller is known. The radio base station includes a single or multiple cells, each of which performs radio communication with a radio terminal. The radio network controller manages multiple radio base stations and allocates a radio resource to radio terminals. Such technology may be referred to as R99 (Release 99).
In recent years, a technology for allocating a radio resource or the like by a radio base station has been proposed for the purposes of improving the throughput, reducing the delay time, and the like. Such technology may be referred to as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Enhanced Uplink (EUL), and the like (for example, 3GPP TS25. 308 V9.1.0, 3GPP TS25.309 V6.6.0).
Here, in an uplink communication, the radio base station assigns a station-specific sequence number (FSN: Frame Sequence Number) to the uplink user data received from a radio terminal. The radio network controller determines a congestion state of a link of the physical layer (TNL; Transport Network Layer) between the radio base station and the radio network controller by checking continuity of the sequence numbers assigned to the uplink data.
In addition, there are known technologies for a radio terminal to hand over from a first radio base station to a second radio base station according to movement of the radio terminal. For example, one conceivable method (hereinafter, referred to as the first method) is that the radio network controller receives the uplink user data from the radio terminal via either the first radio base station or the second radio base station in the handover state by releasing a link between the radio network controller and the first radio base station and then establishing a link between the radio network controller and the second radio base station. Alternatively, another conceivable method (hereinafter, referred to as the second method) is that the radio network controller receives the uplink user data from the radio terminal via both the first radio base station and the second radio base station in the handover state by establishing a link between the radio network controller and the second radio base station while maintaining a link between the radio network controller and the first radio base station.
In the first method, sequence numbers specific to the first radio base station and sequence numbers specific to the second radio base station are not mixed. However, the link between the radio network controller and the first radio base station is released first and then the link between the radio network controller and the second radio base station is established, whereby instantaneous disconnection occurs. In other words, the first method leads to deterioration (reduction) the transmission rate of the uplink user data.
On the other hand, in the second method, the link between the radio network controller and the second radio base station is established while maintaining the link between the radio network controller and the first radio base station, whereby no instantaneous disconnection occurs. Therefore, compared with the first method, the second method can suppress drop in the transmission rate of the uplink user data. However, station-specific sequence numbers assigned by the first radio base station are not associated with station-specific sequence numbers assigned by the second radio base station, and the radio network controller thus receives data assigned discontinuous sequence numbers. As a result, the radio network controller may possibly make erroneous judgment that a link (TNL link) between a radio base station and the radio network controller is congested. Due to such erroneous judgment, the radio network controller transmits a congestion signal (for example, TNL Congestion Indicator) to a radio base station, and the radio base station transmits the transmission rate reduction signal to the radio terminal. In other words, the second method also leads to deterioration (reduction) the transmission rate of the uplink user data.